1. Field of the Invention
The present invention relates to a semiconductor device with a memory circuit, and more particularly to a semiconductor device having at least one read-only memory (ROM) including a plurality of memory cells (bits) on a single semiconductor chip.
2. Description of the Prior Art
Generally, there are two types of read only memory. One is a ROM in which information can be written after a semiconductor chip is encapsulated by a packaging member, and the other is a ROM in which information can not be written after packaging. The ROM of the present invention belongs to the latter type. The ROM of the latter type is called "a mask ROM" and stores information before the chip is encapsulated. Information is written into the ROM by a masking process during semiconductor manufacturing. That is, information is stored in a permanent manner according to a mask pattern.
There are two types of mask ROMs. One is called "a contact ROM" where the bit information to be stored is determined according to whether a memory transistor is connected to a bit (digit) line or not. The ROM of this type has memory transistors at all of the cross points of the address (word) lines and bit (digit) lines arranged in matrix shape, but only the memory transistors selected by the mask pattern are connected to the respective bit lines. It is well known in the mask ROM that a memory transistor acts as an element for varying the potential of a bit line. Therefore, if a memory transistor at a given location designated by an address is conencted to a bit line, the potential of the bit line is varied, while if another memory transistor designated by an address is not connected to a bit line, the potential of the bit line is not varied. Thus, it is determined whether the stored bit information is "1" or "0". In this case a semiconductor device maker must produce a contact mask according to a program or another information provided by a user. However, since the maker can produce memory transistors before the program or other information to be stored in the ROM is provided by the user, the maker can provide a semiconductor device with a mask ROM in which the necessary information has been stored for the user within a short period of time. That is, this type of ROM has an advantage in that the turn around time (TAT) is short. On the other hand, the ROM of this type has a disadvantage in that the ROM is limited to a small capacity since a contact area is required for each bit as described in detail hereinafter.
In contrast, a mask ROM of the other type is not dependent on the contact pattern, but depends on the memory transistor itself. In the ROM of this type, information is stored depending on whether or not a memory transistor for charging (or discharging) the potential of a bit line in response to an address signal is produced at a cross point of a bit line and a word line. If a memory transistor operating in response to an address signal is produced at this point, the potential of the bit line varies, while if a memory transistor is not produced or if a memory transistor does not operate in response to the address signal, the potential of the bit line does not vary. Thus, bit information can be stored in this ROM the same as in the above mentioned contact ROM.
This ROM can store a large quantity of information since the contact area is not required. However, a maker can not produce memory transistors on the chip before a program or another information to be stored is provided by the user, so that a long turn around time is required.
Conventionally, mask ROMs of the above two types are used in accordance with the user's request. Namely, in the case where the quantity of information to be stored is small and where the user desires to use the semiconductor device with the mask ROM as quickly as possible, the former mask ROM is employed in general. In the case of a large quantity of information, that is, a long program, a large table storing predetermined information, or the like, the latter mask ROM is used.
Now, it is assumed that a mask ROM storing 2k words of information (16 bits/word) is required. If the former type ROM is used, one or two weeks are required to provide a semiconductor device with the ROM to the user, however the size of this ROM is as about three times that of the latter type ROM. Therefore, this type ROM is inadequate for large capacity information storage or for semiconductor devices which need circuit elements other than ROMs, such as a microcomputer. On the other hand, though the latter ROM can store about three times as much information as the former ROM, six to eight weeks are required to deliver the semiconductor device to the user. Therefore, the user can not use the device quickly.